Elsevier

Composite Structures

Volume 90, Issue 1, September 2009, Pages 34-42
Composite Structures

Static and dynamic characteristics of multi-cell jointed GFRP wind turbine towers

https://doi.org/10.1016/j.compstruct.2009.01.005Get rights and content

Abstract

An extensive research project is currently being carried out at the University of Manitoba, Canada, involving the development of glass fiber-reinforced polymer (GFRP) wind turbine towers. The towers consist of multi-cell segments, each segment constructed from eight filament wound cells jointed together with resin applied over their interface. The present paper mainly addresses the static and dynamic characteristics, such as failure static loads, modes of failure, fundamental frequencies and periods of such segmented composite towers. Both experimental and numerical results are presented. The experimental investigation involved the testing of two jointed scaled towers. These specimens had a total height of 4.88-m (16-ft) and were tested as cantilevers under static and dynamic loading. The testing was conducted at the W.R. McQuade Structural Engineering Laboratory of the University of Manitoba. Finally, finite element models were developed to analyze the structural behavior, static and dynamic, of single and multi-cell composite segments and towers. The results from the finite element models under static loading were validated through comparison with the experimental results.

Introduction

There is currently a plethora of information available on wind-generated energy. Key organizations such as the European Wind Energy Association (EWEA) and the American Wind Energy Association (AWEA) are reporting extensively on their respective web sites on the enormous increase in wind-generated energy around the world. An overwhelming emphasis on their reports, however, is placed on the environmental and economic benefits of wind energy, which they attribute to the development of new larger and more efficient turbines and rotors. While this may, to some extent, be true, 20% of the total cost of a wind turbine is spent on the construction of the tower [1]. Research, however, to reduce the cost of these towers is almost non-existent. As a result, towers tend to be heavy, difficult to transport to remote areas, and require heavy equipment for their erection.

Although wind turbine towers constitute a major portion of the overall cost of an installation [1], very little research has been conducted to improve their design or to make it easier for wind turbines to be installed in remote or inaccessible places. The most common type of wind turbine towers today are tubular steel towers usually manufactured in sections of 20–30 m with flanges at either end, and bolted together on site. The steel sections are conical (i.e. with their diameter increasing towards the base) in order to increase their strength and, at the same time, to save materials. According to the Danish Wind Turbine Manufacturing Association (DWTMA), it is quite important for the final cost of energy to build towers as optimally as possible [1]. The size of the tower depends on the local wind speed and how this speed varies with height. Other limiting factors include transportation on roads or rail.

Technological advancements over the last 25 years have resulted in significant reduction in the cost of wind-generated energy from 38 US cents in 1982 to between 4 and 6 US cents in 2001 [2]. According to Marsh [3], this dramatic decrease is mainly due to the use of composite materials for the construction of lighter rotor blades. Indeed, composite materials are slowly finding their way into more and more applications in wind generator nacelles, cabins, fairings and parts of towers. Industry estimates suggest that 80,000 tons of finished composites are used annually for blades alone. Composite materials have the potential to decrease the total weight of the wind turbine towers, leading to substantial saving in transportation and erection costs, making wind energy more affordable for remote and rural communities where the number of turbines required is usually small. In a white paper published by Wind Tower Composites [4], it was reported that the cost of composite towers, based on a 2-unit wind farm, is 38% less than the cost of steel towers. For a 25-unit wind farm, the cost of composite towers is 28% less than steel towers. Even though, the cost of composite materials per unit weight is higher than that of steel, the fact that the total weight of the composite towers is lower than steel, results in lower transportation and erection costs. The cost advantage for steel has been eroding over the last five years since the price of steel in the world market has jumped by 250%, while the cost of composite materials has been steadily decreasing. As a result, research in the development of composite wind turbine towers has begun in earnest both in the United States and Europe [5], [6].

Over the last ten years, research has been conducted at the University of Manitoba to develop technologies for winding fiber-reinforced polymer (FRP) poles for use in electrical transmission and distribution networks [7], [8], [9], [10], [11], [12] and towers [13]. The main objective of the research program reported here is to evaluate the structural performance of segmented GFRP wind turbine towers. The design of wind turbine towers is, typically, controlled by the bending capacity, thus, scaled bending tests were performed to evaluate the structural performance of the proposed GFRP segmented towers. The specimens were tested up to failure. The specimens were manufactured using the filament winding process at the FRP Laboratory of the University of Manitoba. A description of the tested specimens, the testing setup, and the instrumentation used are presented in detail in this paper.

Section snippets

Experimental program

The experimental program was carried out in four phases. Phase I involved the testing of two single GFRP cells to obtain the ultimate strength and to observe the mode of failure. In Phase II, single GFRP cells with various lengths were tested under compression. Phase III involved the testing of two GFRP multi-cell towers under static loading. In Phase IV, the two GFRP multi-cell towers were tested under dynamic loading. Standard coupons were also fabricated and tested according to current

Finite element analyses

In the current study, the ANSYS program was used to develop finite element models for simulating the structural behavior of single cells under lateral bending and compression as well as for the eight-cell jointed towers under lateral bending. To model the composite tower, an eight-node quadrilateral layered shell element was used. This element, which is designated by ANSYS as SHELL 99, is a 100-layer shell structure with 6 degrees of freedom per node. It was adopted because of its ability to

Conclusions

Experimental and FEA studies were performed to investigate the static and dynamic behavior of segmented tapered filament wound GFRP jointed wind turbine towers. The most important findings of the present work can be summarized as follows:

  • The average ultimate load carrying capacity and tip deflection of the two towers tested (P3-1 and P3-2) were 19.11-kN and 76.90-mm, respectively. The test results showed a small variation in both ultimate load and stiffness, as indicted by the low coefficients

Acknowledgements

The research project in this study was sponsored by the Natural Sciences and Engineering Research Council of Canada (NSERC); the National Centre of Excellence ISIS Canada; and, the Province of Manitoba. The towers were fabricated at the FRP Manufacturing Facility of the University of Manitoba, and the testing was conducted at the Structural Engineering and Construction R&D Facility at the University of Manitoba. The FEA was conducted at the National Technical University of Athens. The composite

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